The invention relates to a high-frequency comb-line filter, comprising an elongated casing made of an electrically conductive material, conductor rods of an electrically conductive material being arranged in a sequence at predetermined intervals in a substantially integral air cavity within the casing, one end of each conductor rod being short-circuited to the casing and its other end being detached therefrom so that each conductor rod forms with the casing an air-insulated coaxial resonator.
In comb-line filters, the resonators are typically positioned in a sequence one after another so that the inductive and capacitive couplings between the resonators form the couplings between the resonators directly without any separate coupling elements. Therefore, such comb-line filter structures are usually simpler and smaller in structure than conventional filter structures accomplished by separate resonators coupled together, e.g. by separate coils. Comb-line filters can be realized by coaxial resonators, in which case air is used as a medium between the resonators and the electrically conductive casing surrounding the resonators.
FI Patent Application 906215 discloses a comb-line filter in which the medium is air, that is, the filter is air-insulated. In this filter, the resonators consist of electrically conductive conductor rods enclosed in an integral space defined by a single electrically conductive casing. The casing is common to the resonators, forming a coaxial resonator with each resonator.
Today, very stringent requirements are set on the electrical properties of high-frequency filters while increasingly smaller sizes and lower costs of manufacture are also required. In filters accomplished by coaxial resonators, for instance, one aims at an optimal Q factor and the smallest possible losses by using a theoretical optimum value of 3.6, calculated as the ratio between the diameters of the casing and the resonator of the coaxial resonator, and by manufacturing the casing and the resonator of materials having as low losses as possible (materials of high electrical conductivity), such as a copper casing and resonators coated with silver (cf. "Microwave Filters, Impedance Matching Networks, and Coupling Structures", G. Matthaei, L. Young, E.M.T. Jones, Artech House Books, Dedham, MA USA, p. 165-168). However, such materials are relatively expensive, which increases the cost of manufacture for these filters. So one constantly has to compromise between the size, properties and cost of manufacture of the filter.